1. Field of the Invention
The present invention relates in general to an evaporative control in an internal combustion engine adapted to trap fuel vapor in a fuel tank, etc. of a fuel supply system of the engine and supply the vapor together with air to an intake passage, etc. of the intake system and more particularly to a purge gas quantity control in such an internal combustion engine.
2. Description of the Prior Art
A fuel vapor control system has heretofore been proposed in which fuel vapor in a fuel tank, etc. in a fuel supply system of an internal combustion engine is once trapped by a canister and then the trapped vapor is purged from the canister so that the purged air-fuel mixture (purge gas) is supplied by way of a purge line to an intake system of the engine, whereby to prevent the fuel vapor in the fuel tank, etc. from being emitted into the open air, as disclosed in JP-A-62-7962 (Laying-open publication of Japanese patent application).
In the above described fuel vapor control system for supplying the purge gas from the canister to the intake system of the engine, extra purge gas is added to usual air-fuel mixture, so there is a possibility of a large variation in air-fuel ratio due to the supply of the purge gas. The purge gas supply quantity is thus controlled so that its influence over the injection quantity Ti of fuel supplied to the engine is constant, e.g., the percentage of the purge gas quantity relative to the fuel injection quantity Ti is equal to or less than 10% or so. Specifically, a purge gas quantity is determined so as to have a predetermined ratio relative to a basic fuel injection quantity T.sub.P or the like engine operating condition, and the width of pulse for drive of a purge control valve serving as a purge gas quantity altering means is controlled so that the purge gas quantity determined as above is attained.
However, with the structure adapted to control the purge gas quantity in accordance with the basic fuel injection quantity T.sub.P, the purge gas quantity is caused to decrease as the vehicle goes to a higher altitude, and it becomes impossible to attain a required purge gas quantity, resulting in that fuel vapor is escaped from the canister and hydrocarbons HC are emitted into the open air. Due to this, there exists a problem that the emission control standards having become more stringent recently cannot be met.
In this instance, the reason why the purge gas quantity reduces as the vehicle goes to higher altitudes is as follows. That is, consider a case in which a vehicle whose engine is conditioned so as to meet the requirement for the purge gas quantity at flatlands or low altitudes, goes to highlands or higher altitudes. The purge gas quantity is firstly determined by the difference of the pressures across the purge control valve (P.sub.P -P.sub.E) and the opening area of the purge control valve (i.e., drive pulse width).
When going to higher altitudes, the atmospheric pressure PA becomes lower. Due to this, when the same basic fuel injection quantity T.sub.P as that at flatlands is given, the pressure P.sub.E downstream of the purge control valve becomes higher. Thus, assuming that the pressure P.sub.P upstream of the purge control valve is constant, controlling the purge control valve by the same drive pulse width causes the purge gas quantity to be reduced.
On the other hand, the pressure within the fuel tank is determined by the check valve for the fuel tank and the atmospheric pressure, so the pressure within the fuel tank becomes lower at higher altitudes, and also the pressure P.sub.P upstream of the purge control valve becomes lower.
As a result, the differential pressure (P.sub.P -P.sub.E) across the purge control valve becomes smaller, and thus the purge gas quantity is reduced.